(1) Field of the Invention
The present invention relates to an exhaust emission control system for a vehicle internal combustion engine. In particular, the present invention relates to an exhaust emission control system capable of improving an exhaust emission purifying performance while ensuring a braking force by a vacuum type brake booster.
(2) Description of the Related Art
Conventionally, an exhaust emission purifying technique utilizing reactions on a catalyst has been known as a technique for reducing harmful substances (e.g., smoke and NOX as well as unburned substances such as HC, CO, and H2) in exhaust gases emitted from a vehicle internal combustion engine.
However, the exhaust emission purifying technique has the problem that harmful substances such as HC are emitted into the atmosphere before the catalyst is activated. To address this problem, a technique for promoting activation of a catalyst has been developed. For example, for a cylinder injection internal combustion engine, a two-stage combustion technique (or two-stage injection technique) has been developed which injects fuel in two stages such that sub-injection is carried out from an expansion stroke after main injection (at a lean air-fuel ratio) is carried out, and causes oxygen remaining after the main injection and unburned substances left after the sub-injected to react with each other in an exhaust system (extending from a combustion chamber to a catalytic converter), thereby reducing harmful substances or promptly activating a catalyst.
On the other hand, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 3-117611 and Japanese Laid-Open Patent Publication (Kokai) No. 4-183921 for example, a technique has been developed which enables prompt activation of a catalyst by increasing the exhaust pressure at the time of cold-start of an engine.
Further, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-027145 for example, a technique has been developed which enables prompt activation of a catalyst at the time of cold-start of an engine by carrying out two-stage combustion and increasing the exhaust pressure (suppressing the flow of exhaust).
Incidentally, a vehicle is ordinarily equipped with a vacuum type brake booster that supplements a braking force generated by the driver. The vacuum type brake booster is capable of supplementing the braking force by utilizing negative pressure developed in an air intake system of an internal combustion engine when the vehicle is braked.
If the exhaust pressure is increased as mentioned above, however, the exhaust resistance is raised to increase the intake air volume, so that the negative pressure in the air intake system approximates the atmospheric pressure. Also, if the two-stage combustion is carried out, a throttle valve is opened in order to set the air-fuel ratio in the main injection to a lean air-fuel ratio, so that the negative pressure in the air intake system approximates the atmospheric pressure. If the negative pressure in the air intake system approximates the atmospheric pressure, it is impossible to ensure a sufficient negative pressure to be used by the vacuum type brake booster and to sufficiently supplement the braking force.
Accordingly, it may be considered that the two-stage combustion and the exhaust pressure increase (the suppression of the exhaust flow) are prohibited in the case where the detected negative pressure in the air intake system approximates the atmospheric pressure and a desired negative pressure cannot be ensured.
However, the two-stage combustion and the exhaust pressure increase (the suppression of the exhaust flow) are carried out mainly at the time of cold-start of an internal combustion engine. On this occasion, the vehicle is usually stopped, and no problem occurs even if a braking force is not sufficiently supplemented. In such a case, it is not preferred that the two-stage combustion or the exhaust pressure increase (the exhaust flow suppression) is not carried out, because harmful substances such as HC are emitted into the atmosphere to deteriorate an exhaust emission purifying performance.
It is therefore an object of the present invention to provide an exhaust emission control system for a vehicle internal combustion engine, which is capable of ensuring a braking force by a vacuum type brake booster at the time of braking and improving an exhaust emission purifying performance.